I have no idea what “Use WebXR (WebVR, WebAR, Magic Window, Etc.) - Babylon.js Documentation” is. Looks like an old copy of the real site.
The current docs have examples on the right side. I think your code might be blowing up because it is backwards. My code tgets the input from the helper, not passing a BABYLON.WebXRDefaultExperience to a constructor.
let xrInput = defaultExperience.input;